1. Field of the Invention
The present invention relates to a motor apparatus comprising a plurality of axially connected unit motors and thus having a large axial length as compared with the diameter. More particularly, the invention is concerned with a submersible motor suitable for use in driving a pump which is submersible into a deep well.
2. Description of the Prior Art
In order to diminish a pump system on the ground, it is conventional to pump up underground water in a well by means of a submersible pump installed at the bottom of the well and coupled to a submersible motor. In this type of pumping system, however, the pump must work to provide a considerably high delivery pressure when the depth of the well is large and, thus, requires submersible motor apparatus of a large output. A large-sized high output motor apparatus needs a well of a large diameter. It requires not only a high degree of technology but also a huge cost to enlarge the diameter of an already constructed well or to sink a new well of a large diameter. This is particularly true with a deep well. Therefore, in the case where the well has a limited diameter, the motor which can be used with the well and provide a required output must be of a large axial size as compared with the diameter of the well. Such a type of submersible motor, however, has stator and rotor cores of increased thicknesses which extremely lowers the working property of the stator and rotor, particularly, the coil winding operation of the stator and die-casting of the rotor. Additionally, the increased thickness of the rotor core tends to cause deflection and oscillation of the rotatable part of the motor thereby resulting in a motor which is extremely unstable from a mechanical point of view.
In order to facilitate the production of a submersible motor, it has been proposed and practised to axially divide the stator and the rotor of the motor into a plurality of sections, i.e. into a plurality of unit motors which are connected in series to constitute a submersible motor apparatus. In many cases, the upper shaft end of the uppermost unit motor is used as the output shaft of the motor apparatus, to which the shaft of the submersible pump is connected. The lower shaft end of the lowermost unit motor is associated with a thrust bearing for the axial thrust applied by the submersible pump and all the unit motors.
The submersible motor having the described construction and arrangement naturally produces a considerable amount of heat during its operation to cause a much greater temperature differential between the inner and outer walls of the motor than encountered by ordinary motors used in the air, because the casing of the submerged motor is cooled by the ambient water and maintained at a temperature substantially equal to 20.degree. C. The temperature differential between the inner and outer portions of the motor results in a thermal expansion of the rotor shaft of the motor. Assuming that a submersible motor has an overall length of 2 m, the temperature differential between the inner and outer portions of the motor is 50.degree. C., and the coefficient of thermal expansion of the rotor shaft is 1.6.times.10.sup.-5 mm/.degree.C., the thermal expansion d of the shaft in the axial direction is calculated as follows: EQU d=2,000.times.1.6.times.10.sup.-5 .times.50=1.6 (mm)
Thus, the rotor shaft exhibits an axial thermal expansion of 1.6 mm when the motor is operated continuously. Since the lower end of the motor shaft is born by a thrust bearing, the axial thermal expansion of the shaft is transmitted to the shaft of the submersible pump to lift the pump runner to thereby vary the gap between the pump runner and the pump casing, resulting in various inconveniences such as decrease in the performance of the pump and overload on the motor due to a change in the characteristic of the pump. It is conceivable that the pump runner could directly contact the pump casing due to the lifting of the pump runner. In such a case, not only is the submersible pump seriously damaged but also an extraordinarily heavy load is dangerously applied to the thrust bearing.
It is also to be pointed out that the axial thermal expansions of the unit motors are accumulated to cause an axial offset of the magnetic center of the rotor from the magnetic center of the stator in each unit motor, with the axial offset being larger in the superposed unit motor than in the underlying unit motor. This axial offset of the magnetic center of the rotor from the magnetic center of the stator produces an axial component of the magnetic attractive force between the rotor and the stator. This axial force component is effective to increase the load applied onto the thrust bearing to thereby shorten the operative life thereof.
In contrast to the above-stated submersible motor having a thrust bearing disposed at the lower end of the rotor shaft, there has been proposed another type of motor apparatus in which a thrust bearing for receiving the axial load on the submersible pump and all unit motors is provided at the upper shaft end of the uppermost unit motor. While this type of motor can prevent the axial thermal expansion of the rotor shaft from being transmitted to the shaft of the submersible pump, the motor needs a thrust bearing of an increased diameter because the thrust bearing must be provided therein with a bore through which the motor shaft extends. The increase in the thrust bearing diameter results in the increase in the diameter of the submersible motor apparatus. In addition, the thermal expansion of the rotor shafts of series-connected unit motors are accumulated in downward direction, so that the underlying unit motor exhibits a larger axial offset of the magnetic center of the rotor from the magnetic center of the stator than in the superposed unit motor, with a result that the output power of the submersible motor apparatus is decreased, which, in turn, tends to cause the motor apparatus to be overloaded.